The life story of a jellyfish.

The Life of a Jellyfish

From a continuing series on revolting creatures.

A really good place to have a meaningful and pain-free relationship with jellyfish is the Monterey Bay Aquarium. Behind glass, artfully illuminated, the creatures are the very definition of elegance—beautiful in movement and appearance. People stand quietly, mesmerized, watching the translucent bells pulse at a hypnotic 30 beats per minute, a little slower than Lance Armstrong's heart rate.

Encounters without a glass wall—dangling tentacle to defenseless skin—are not so happy. August is the month. (January if you're Aussie.) Don't worry, there's still time: You might just as well get stung over Labor Day weekend.

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A profusion of jellyfish is often described as an invasion or an attack. Which is laughable, given the guiding principle of jellyfish behavior—"whatever." No brain, no spine; they don't have the capacity to plan a beach invasion. We bump into them, and because we're too big to eat, they perceive us as attackers.

Planning is not their forte. In place of a brain, jellies have a nerve net. Jellyfish are the free-floating relatives of sea anemones and corals, much older than fish, and not much changed for more than 600 million years. They ruled the ocean, in their passive way, when there was almost nothing but ocean. Now they drift into their food or their food drifts into them. The pulsing creates a current that pulls prey within reach.

Depending on the kind of jellyfish, its sting can be minor, memorable, or, in very rare cases, fatal. (More than 1,000 species of jellyfish have been described. Like trees in the Amazon rainforest, there are probably thousands more still undiscovered.) The stinging cells—called nematocysts—are embedded in the tentacles and the umbrella of the jellyfish. The nematocyst generates a dart; the dart gets fired into prey, delivering a cocktail of neurotoxins, and then falls off the jellyfish. The nematocyst then goes to work generating a new dart for the next victim. Sometimes a tentacle or glob of jelly mucus breaks off and the slimy transparent stuff sticks to human skin, which is, to put it mildly, disconcerting.

Enough about our feelings: What's it like to be a jellyfish? Is it boring to be a drifter? "All the medusa has to do," says Monty Graham, marine scientist at the Dauphin Island Sea Lab in Alabama and convener of the first International Conference on Jellyfish Blooms, "is eat and have sex. I don't call that a bad life." (The floating stage of the jelly, with its dangling tentacles, is named for the snake-haired monster.)

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It's not well understood how the jellies find each other for mating. "Don't think of jellyfish trying to seek each other out," Graham said. "Think of it in terms of avoiding being carried away from each other." Staying close to potential partners involves the following three talents: They have the ability to sense light and dark, though they have no eyes. They perceive gravity and can tell up from down. They can position themselves in relation to currents. Once they've sensed the right placement, the males release sperm from their mouths into the ocean and hope for the best—another whatever/wherever behavior. The odds are good because males and females are in a swarm; clouds of sperm and eggs swirl in the water. The moon jellyfish, the pretty pearly one common along our coasts, is a little more focused. The male sends his sperm out in a long string—the length of a strand of spaghetti. The female catches it and eats it. Happily for the survival of the moon-jelly race, her mouth is the route to her eggs.

The fertilized moon-jelly eggs cling to pits on the female's oral arms until they turn into swimming larvae. Whatever degree of parenting they receive, moon-jelly quality or not, fertilized eggs become larvae about .5 millimeter long and settle on rocks to become polyps, barely visible, like small and colorless sea anemones. (The unlucky ones sink to the ocean floor where they may be covered with sediment and have a harder time.) Eventually the polyps clone into layers, resembling a miniature stack of Frisbees. The one at the top begins to pulse and then flies off, followed over a week or two by 30 or so of his or her siblings. The polyp stage can last for years, cloning and sending out more medusas at irregular intervals. The medusas don't live much more than a year.

Jellyfish may meet a violent death in the beak of a sea turtle, the mouth of a fish, or the bill of a seagull. The tuna and swordfish that eat them have been overfished, but in the Gulf of Mexico, a Volkswagen-sized beast, the Ocean sunfish, specializes in feeding on jellies. So does the leatherback turtle. One of the many mysteries of ocean life is how the sunfish and sea turtle can grow as big as 2,000 pounds on a diet of creatures whose bodies are 95 percent water.

If they escape predators, jellyfish die not long after sex. Their bells turn raggedy, the tissues that once were clear become opaque, and they drop to the ocean floor.

People who watch the sea—lifeguards, surfers, and fishermen—believe that there are more jellyfish than ever. Though this has definitely been the summer of beach closings, marine biologists find it hard to pin down the numbers. It could be, the more cautious say, that we're seeing more near beaches this year because of changing currents or shifting winds, or we could be at some point in a climatic cycle that causes jelly populations to increase before they wane again. It's impossible to map the long-term ebbs and flows because we don't have good data on jellyfish blooms in past decades or centuries. Census and tracking are difficult because it's very, very hard to put a tag on a jellyfish, they have a short life span, and no one cares to come back and recapture them. (It's even hard to net them without having them fall to pieces.)

It's easy to assume that global warming must be involved in the apparent jellyfish explosion. After all, we bump into these scary floaters in the summer. But they're around in numbers in the summer because there's plenty to eat then. (There are also jellyfish floating under Arctic and Antarctic ice.) It's true that jellyfish might make more medusas at higher temperatures, but recent experiments showed that each juvenile medusa has a lower chance of survival in warmed water.

We do know for certain that changes in food supply can cause a population increase. Jellyfish consume tiny fish, shrimp larvae, and plankton. (Some jellyfish specialize in eating other jellies; these cannibals include the extremely useful Aequorea, the source of a luminescent substance—green fluorescent protein—used as a marker by virtually every molecular-biology lab in the world.) Fertilizer and sewage runoff from our coastal cities and towns puts nutrients in the water that fertilize the tiny plants—phytoplankton—that are eaten by the small marine animals—zooplankton—that are the primary jellyfish food. Jellyfish have always hovered near the coast; we may be inadvertently beckoning more of them closer with our organic waste. And overfishing and pollution have greatly decreased the jellyfish's main competitors for food—the little 4- to 8-inchers like anchovies, sardines, and menhaden.

We don't know enough now to predict the future of jellyfish populations; we do know we don't want them at our beach. Still, there's something captivating about the creatures, even outside an aquarium. E.O. Wilson, an ant expert and the greatest environmental scientist of our time, begins his autobiography with a boyhood reminiscence of seeing jellyfish in the waters of Northwest Florida. That jellies are not deep thinkers doesn't diminish their appeal. Despite the appearance of passivity, there's some precise and efficient hard wiring at work that has kept them around for millions of years. In polluted parts of the ocean, they may be the last ones standing—or, rather, floating.